Abstract

As the world energy demand increases, and the desire for cleaner fuels strengthens, a number of major oil and gas companies are developing Floating Liquefied Natural Gas (FLNG) vessels to harvest natural gas ‘stranded’ in reservoirs that have previously been considered too uneconomic to develop. A key requirement for this new generation of vessels is a high volume of low temperature seawater for process cooling.
The aim of this research is to investigate whether the concepts underpinning free hanging cantilever seawater intake risers used on Floating, Production, Storage and Offloading (FPSO) vessels can be extended to the design of seawater intake risers for FLNG vessels in order to reach and import colder seawater from depths greater than has so far been achieved with these systems. The research focusses on establishing the physical, mechanical and fatigue properties of a number of material elements under consideration for this application and then investigates a number of combinations to determine the optimum configuration for a hybrid deep seawater intake riser.
To demonstrate the strength and fatigue capabilities of the hybrid riser, the selected configuration is then subject to a more detailed analysis with consideration of a number of key aspects such as vessel motion, marine growth, vortex induced vibration, stability due to internal flow and excursion due to external fluid. A number of sensitivities are also performed with respect to riser damping, riser length, vessel size and geographical location. Additionally, the flow characteristics in terms of pressure loss and temperature gain are examined and a number of sensitivities performed to show that the cold seawater can be imported effectively. Finally, using published data for FLNG vessels currently under construction, an economic argument is presented to highlight the potential cost advantage of reaching and importing colder seawater by means of a deep seawater intake riser.
As a result of this research, the solution being presented offers a significant technological advantage for these systems in the field enabling high volumes of seawater to be imported from greater depths whilst accommodating the loads induced by the environmental conditions and minimising the loads induced into the hull of the vessel. Furthermore, the solution is based on the concepts of a field proven system, thereby limiting the risks associated with untested technological advancements. The findings of this research enable the process efficiencies of FLNG vessels to be greatly enhanced thus contributing to the more efficient extraction of a cleaner fuel which, in a world with ever increasing energy demands, is critical to the global economy.
The novelty of the research is demonstrated by two successful patent applications, one in relation to the improved features of existing seawater intake riser systems and the other in relation to the use of multiple material elements for a hybrid seawater intake riser. Both patents have been examined and granted in five jurisdictions, namely, Europe, Japan, China. South Korea and the USA.